The technique of rapid alteration of extracellular composition of a frog ventricle strip, during a single, maintained, microelectrode implement, has been used to study the calcium dependence of electromechanical coupling: The voltage-tension relationship. It has been possible, in most preparations, to markedly alter composition during the diastolic period between one beat and the next (or between one clamp pulse and the next) to a degree close to that of the ultimate steady state change in concentration. This technique will be applied to mammalian ventricular trabeculae, where extracellular access is slower. In particular, the mechanisms involved in the delayed inotropic effects of membrane depolarization will be studied by programmed voltage perturbations and extracellular composition changes, recording contractile force and membrane currents during voltage clamp.